1. Field of the Invention
The present invention relates to a color cathode ray tube, and more particularly, to a shadow mask frame assembly which allows an electron beam emitted from an electron gun to be scanned on a fluorescent film accurately.
2. Description of the Related Art
Referring to FIG. 1 showing a conventional color cathode ray tube (CRT), a fluorescent film 11 is formed on the inner side of a panel 10, and a shadow mask frame assembly 20 is spaced a predetermined distance from the fluorescent film 11. The shadow mask frame assembly 20 is constituted by a shadow mask 21 facing the fluorescent film 11 and a frame 22 for supporting the shadow mask 21. The panel 10 is adhered to a funnel 30 comprising a neck portion 31 having an electron gun 40 mounted therein and a cone portion 32 having a deflection yoke 50.
The shadow mask 21, as shown in FIG. 2, includes a mask plate 21a having a plurality of electron beam passing holes H, a rim 21b extending from the periphery of the mask plate 21a, a skirt portion 21c extending downwardly from the rim 21b at a right angle. The frame 22 includes a supporting wall 22a to which the skirt portion 21c of the shadow mask 21 is welded for support, and a flange 22b inwardly extending from the supporting wall 22a.
The electron beams emitted from the electron gun 40 mounted in the neck portion 31 (see FIG. 1) are selectively deflected by the deflection yoke 50 and pass through the electron beam passing holes H of the shadow mask 21 to land on the fluorescent film 11 so that phosphors are excited.
Only about 15.about.30% of the electron beams emitted from the electron gun 40, that is, thermions, pass through the electron beam passing holes H of the shadow mask 21. The remaining electron beams impinge on the mask plate 21a of the shadow mask 21 so that the shadow mask 21 and the frame 22 supporting the shadow mask are heated so that they are thermally expanded.
When the shadow mask 21 and the frame 22 are thermally expanded, since their thermal expansion coefficients are different from each other, the radius of curvature of the mask plate 21a changes over time. That is to say, in the beginning stage of operating the CRT, the shadow mask 21 is heated, and expands prior to the frame 22, so that the radius of curvature of the mask plate 21a becomes smaller, which is called a doming phenomenon. As time passes, the frame 22 is thermally expanded so that the radius of curvature of the mask plate 21a increases again.
To reduce the doming phenomenon due to thermal expansion of the shadow mask 21, conventionally, a buffer groove 21d is formed on the rim 21b of the shadow mask 21 to compensate for the expansion of the mask plate 21a, or the radius of curvature of the shadow mask 21 is made larger.
However, since the shadow mask 21 is supported by the frame 22 such that the skirt portion 21c is fixed to the supporting wall 22a of the frame 22, there is a limit in compensating for the expansion of the mask plate 21a. Also, since the skirt portion 21c and the buffer groove 21d must be formed, the fabrication of the shadow mask frame assembly is burdensome and costly.